The present invention relates to the processing and drying of semiconductor wafers or similar items at high pressures.
Wet chemical processes are a crucial part of semiconductor device fabrication. Such processes include etching of films, removal of photoresist, and surface cleaning. Over the years, specific applications have spawned the development of numerous chemistries for wet processing, including APM (a mixture of ammonium hydroxide, hydrogen peroxide, and water), HPM (hydrochloric acid, hydrogen peroxide, and water); SPM (sulfuric acid and hydrogen peroxide), SOM (sulfuric acid and ozone), and others for specific cleaning or etching tasks. Many of these chemistries are used at or near their boiling points, since chemical reactivity, and therefore the effectiveness of the cleaning, is a function of temperature. Recent developments in wet processing technology have incorporated the use of various gases with aqueous or other liquid solutions to accomplish a desired process objective. For example, the use of ozone and water creates a strong oxidizing solution that may be useful in semiconductor processing. The use of hydrochloric acid or ammonia gas injected into water to create a low or high pH solution with specific properties are additional examples of the use of gas technology.
The use of gas/liquid process mixtures is often limited by gas solubility and temperature constraints. Solubility limitations are heightened when aqueous solutions are used. The limited solubility of gases such as ozone in water at ambient conditions, for example, limits the effectiveness of ozone/water solutions for oxidizing organic compounds, as there is simply not enough ozone available to promote the oxidation process. Reactivity constraints related to temperature are often intertwined with solubility limitations. For example, the solubility of virtually all gases in liquid solution decreases with increases in temperature. Chemical reactivity, however, increases with increasing temperature. These two factors are in conflict with each other for process optimization. Additionally, many of the aqueous solutions used in semiconductor processing are limited by their boiling points. One reason it is desirable to avoid boiling is to prevent cavitation and suppress bubble formation for more effective use of megasonic waves in cleaning wafer surfaces. For example, a 5:1:1 mixture of water, ammonium hydroxide, and hydrogen peroxide will boil at approximately 65 C. Accordingly, such a mixture cannot be maintained in liquid form at elevated temperature unless the composition is changed to elevate the boiling point.
A critical step in the wet-processing of semiconductor device wafers is the drying of the wafers. Any rinsing fluid that remains on the surface of a semiconductor wafer has at least some potential for depositing residue or contaminants that may interfere with subsequent operations or cause defects in the end product electronic device. In practice, deionized (xe2x80x9cDIxe2x80x9d) water is most frequently used as the rinsing fluid. Like most other liquids, DI water will xe2x80x9cclingxe2x80x9d to wafer surfaces in sheets or droplets due to surface tension following rinsing. An ideal drying process would operate quickly to effect the removal of these sheets or droplets and leave absolutely no contaminants on the wafer surfaces, while presenting no environmental or safety risks.
Although various technologies have been used to dry wafers and reduce the level of contaminants left on the wafer surface after drying, the most attractive technology currently available falls under the broad category of surface tension trying. A typical surface tension dryers accomplishes wafer drying using the following steps: (1) wafers are immersed in a rinse medium; (2) the rinse medium is either drained away from the wafers or the wafers are lifted out of the rinse medium, exposing them to a displacement medium that is typically an inert carrier gas containing a percentage of organic vapor, usually an alcohol, such as isopropyl alcohol (xe2x80x9cIPAxe2x80x9d); (3) the organic vapor dissolves in the surface film of the rinse medium, creating a concentration gradient in the liquid, which in turn creates a surface tension gradient that enables the higher surface tension in the bulk liquid to essentially xe2x80x9cpullxe2x80x9d the lower surface tension liquid away from the wafer surface along with any entrained contaminants to yield a dry wafer; and, in some instances, (4) the displacement medium may be purged from the locale of the wafer using a drying medium such as an inert gas stream. Additionally, the carrier gas may be heated to assist in drying and to prevent liquid condensate from forming on the wafer surfaces.
Conventional surface tension drying technology is limited by at least the following factors: (1) it involves the inherent hazard of causing IPA, a flammable liquid, to be boiled at a temperature well in excess of its flash point; (2) it requires the consumption of IPA at relatively high rate; and (3) it creates relatively high fugitive organic vapor emissions.
In light of the limitations inherent to these and other processing and drying technologies, it is an object of one aspect of the present invention to suppress the boiling point of a wafer processing liquid to permit processing at elevated temperatures.
It is an object of another aspect of the present invention to increase the solubility of gases in the liquid phase to enhance chemical reactivity.
It is yet another object of the present invention to prevent cavitation and suppress bubble formation for more effective use of megasonic waves to enhance cleaning performance.
It is still another object of the present invention to reduce or eliminate the need for using an organic vapor as a drying or displacement medium in a wafer drying process.
The term xe2x80x9cwaferxe2x80x9d means a semiconductor wafer, or similar flat media such as photomasks, optical, glass, and magnetic disks, flat panels, etc.
To these ends, in a first aspect of the invention, a method of drying a wafer includes placing a wafer into a vessel, immersing the wafer in a liquid, pressure-sealing the vessel, pressurizing the vessel, and then controlling removal of the liquid. Thereafter, the pressure may be reduced in the vessel and the dry and clean wafer may be removed.
The drying process operates at a pressure preferably between 10 and 100 atmospheres, and more preferably, between 20 and 50 atmospheres. The gas delivered to the vessel is advantageously also temperature controlled. The high pressure promotes the dissolution of gas into the liquid, generating a concentration gradient and a related surface tension gradient. As the liquid along the surface is drained away to expose fresh liquid to the gas, the gas preferably continues to be supplied to maintain the surface tension gradient. The surface tension gradient pulls liquid from the surface of the wafer as the liquid level descends, yielding a clean, dry wafer.
In second aspect of the invention, in a method for processing a wafer, high pressure is used to raise the liquid boiling point allowing processing at higher temperatures, to increase reactivity. The method may advantageously use a variety of liquids and gases to achieve specific objectives.
In a third aspect of the invention, megasonic waves are used in conjunction with pressurized fluid to yield enhanced cleaning performance with higher efficiency.
In a fourth aspect of the invention, supercritical substances are provided in a sealed vessel containing a wafer to promote cleaning and other treatment.
In a fifth aspect of the invention, an apparatus for processing wafers at high pressures is provided. Preferably, the apparatus includes a pressure sealable vessel, a floating or hinged moveable drain within the vessel, and orifices for adding liquid and gas to the vessel.
In a sixth aspect of the invention, phase changes between liquid phase and critical phase are used to process a wafer.